Handle fully-qualified syntax in intra doc links

[Manishearth]

Related: #62834

#74489 makes it so that Type::Item works for trait items. There currently is anchor ambiguity between multiple traits that share an item, so it's not worth disambiguating further, however if we can fix that It would be nice if <Type as Trait>::Item could link to the appropriate bit in the type's impl section.

This would involve parsing the path as two paths, perhaps just using rustc_ast and parsing it as a TyKind::Path.

[jyn514]

This should also handle <dyn T>::f (found while working on tokio-rs/tracing#940):

/// [T::f]
pub trait T {}

impl dyn T {
    pub fn f() {
    }
}

warning: unresolved link to `T::f`
 --> tmp.rs:1:6
  |
1 | /// [T::f]
  |      ^^^^ the trait `T` has no associated item named `f`